Migrate from Bintray to OSSRH

[neiser]

As Bintray is sunsetting, migrate to OSSRH, see this guide and

• https://github.com/sonatype/nexus-maven-plugins/tree/master/staging/maven-plugin
• https://medium.com/codestory/how-to-publish-artifacts-on-maven-central-24342fd286cd

See also qaware/sonarqube-build-breaker#10

[neiser]

Sucessfully test with ./release.sh 1.1.2, which replaces the manual release process and removes the dependency to Bintray. Artifacts are also visible in official OSSRH repo: https://oss.sonatype.org/#nexus-search;quick~openapi-generator-for-spring-starter

Also snapshots are now deployed from the build pipeline. I consider this issue done.